Symposium on Diseases of the Liver
The Clinical Significance of Hepatitis B Virus Antigens and Antibodies Paul V. Holland, M.D.,* and Harvey J. Alter, M.D.**
Perhaps the most striking aspect of the hepatitis B surface antigen (HBsAg, Australia antigen) has been the rapidity with which this basic research discovery has found widespread clinical application. This antigen serves as a marker for the type B ("serum") hepatitis virus and as such has been invaluable in the prevention of post-transfusion hepatitis, in the diagnosis and epidemiologic investigation of sporadic hepatitis, and, potentially, in the development of a vaccine for type B hepatitis. These clinical applications provide the framework for this article. Nomenclature and Current Concept of the Hepatitis B Virus The Australian antigen has been shown to be associated with only one form of human hepatitis virus, namely the hepatitis B virus (HBV), previously known as the serum hepatitis virus. Further, it has been shown to reside only on the surface of this virus and hence its current designation, hepatitis B surface antigen (HBsAg). The hepatitis B virus has, in addition, an internal component or core (Fig. 1) which has its own antigenic determinant, now known as hepatitis B core antigen (HBcAg). The corresponding antibodies are designated anti-HBs and anti-HBc respectively. The core of the HBV appears to be produced in the nucleus of liver cells;12.23 it contains DNA polymerase,26 a virus-specific enzyme, and may also contain double-stranded DNA,4 suggesting that it is the essential replicative element of the HBV. Cores are apparently released into the cytoplasm where they are enveloped by coat material containing HBsAg, forming the complete HBV particle or Dane particle5 (Fig. 1). An enormous excess of coat material is produced and released unassembled (Le., without the core component) into the blood stream where it circulates as small spheres and some associated tubular forms (Fig. 1). Thus, the vast majority of antigen measured by tests for HBsAg represents unassembled or incomplete virus. Clinical studies suggest, however, that whenever HBsAg is detected, some complete, infectious virus particles are also present. 20 *Chief. Blood Bank Department, Clinical Center, National Institutes of Health, Bethesda, Maryland **Chief, Immunology Section, Blood Bank Department, Clinical Center, National Institutes of Health, Bethesda, Maryland
Medical Clinics of North America- VoL 59, No. 4, July 1975
HOLLAND AND HARVEY
Figure 1. Electron micrograph of particles from an HBsAg positive serum (magnifica. tion 90,000 x 2). A is the 42nm Dane form with a clearly defined outer coat and an inner core, B is the 15 to 22 nm spherical form , and C is the filamentous form which is 15 to 22 nm in diameter.
Association of HBsAg with Only Type B Hepatitis The association of HBsAg with both "serum" and "infectious" hepatitis derives from the inaccuracy of clinical diagnosis. Classically, serum hepatitis has been diagnosed only when a history of recent transfusion or other known inoculation was obtained. All other cases were labelled "infectious hepatitis" even when they lacked evidence of epidemic spread. Based on the detection of HBsAg and anti-HBs, it is now evident that these non-epidemic hepatitis cases (sporadic hepatitis) are largely due to infection with HBV despite the absence of a known parenteral exposure. The specificity of HBsAg for type B hepatitis is indicated by its strong association with transfusion15 and needle-spread hepatitis10 and its almost universal absence in epidemics of typical infectious hepatitis.9 In addition, successive episodes of hepatitis in transfused patients4 and cross-immunity experiments28 have strongly suggested the presence of at least two distinct hepatitis viruses, only one of which is HBsAg-associated. Lastly, one can now demonstrate an antigen on a virus-like particle in the stool of patients with infectious hepatitis (type A) which is serologically distinct from HBsAg.14 In point source hepatitis epidemics, antibody seroconversion to this hepatitis A antigen has been documented whereas antibody response to HBsAg was absent. 14 Tests for HBV Antigens and Antibodies Counterimmunoelectrophoresis17 (CEP, CIEP) has been the major test employed for routine HBsAg screening. While not especially sensitive, it is rapid, relatively inexpensive and can detect HBsAg in the majority of sera in which it is present. A new, very rapid test of slightly
VIRUS ANTIGENS AND ANTIBODIES
greater sensitivity is the latex agglutination test.35 Still more sensitive are reversed passive hemagglutination (RPHA)B and radioimmunoassay (RIA)31 tests; these, however, are somewhat more costly and more difficult to perform. Radioimmunoassay is the most sensitive, the most objective and the most specific of current tests. All tests for HBsAg have a low but definite rate of false positive results; positive results must be confirmed by an independent method or by appropriate specificity testing. The simplest and most available tests for anti-HBs (CEP, gel diffusion) are so insensitive as to miss all but the highest titered sera. Hemagglutination44 and radioimmunoassay29 tests, which are on the order of 1000 times more sensitive, have made it possible to demonstrate seroconversion to HBsAg in the vast majority of type B hepatitis cases. Assays for HBcAg and anti-HBc 2. 22. 37 are currently available only in research laboratories.
Subtypes of HBsAg HBsAg is composed of a group reactive antigenic determinant, a, found in all HBsAg-positive sera and additional subdeterminants, d-y and w-r, which behave as allelic pairs.30 All HBsAg-positive sera can thus be divided into four major subtypes: HBsAg/adw, HBsAg/ayw, HBsAg/adr, and HBsAg/ayr. The major significance of the subtypes of HBsAg is that they are virus determined and thus may signify strains of HBV.21 When HBsAgpositive blood of one subtype is transfused, the same subtype is detected in the new host; when chronic carriers of HBsAg are serially tested, the subtype remains constant throughout their clinical course. The subtypes of HBsAg are important epidemiologic tools, especially since they have different distribution ratios in various parts of the world. 30 They may also play a role in the development of a hepatitis B vaccine; while it is known that an individual is resistant to rechallenge with HBV of the same HBsAgsubtype,27 it is not yet clear that he would be immune to challenge with HBV of a different subtype.
Significance of HBsAg The presence of HBsAg indicates that an individual is about to develop acute viral hepatitis type B, that he already has acute or chronic type B hepatitis, or that he is an asymptomatic carrier of HBV. The blood of an individual who is HBsAg-positive should be considered infectious whether or not he is clinically symptomatic.20 He should not donate blood or share razors, toothbrushes, needles, etc. HBsAg has also been found in body secretions, including saliva and semen. Whether these secretions contribute to hepatitis B transmission has not been established. In addition to its use as a marker for the presence of HBV, HBsAg may serve as a prototype vaccine until tissue culture of HBV is possible. It has been shown that heating of HBsAg in serum can remove infectivity while retaining immunogeriicity; Krugman et alP demonstrated the usefulness of this product when given before challenge with known infectious serum (MS-2).
HOLLAND AND HARVEY